The invention relates to an apparatus for preparing beverages, comprising: a reservoir for holding water, an electric heating element for heating the water, means for the transport of heated water to an outlet opening, a thermostat for turning on and turning off the heating element, and means for measuring an off period of the heating element.
Such an apparatus, particularly a coffee maker, is known from the European Patent Application EP 0 171 619. Said known coffee maker is of the flow-type having its reservoir connected to the outlet opening via a flow pipe which is heated by the heating element. The cold water from the reservoir is heated in the flow pipe by means of the heating element and is forced to the outlet opening where the hot water comes into a coffeepot via a filter holder with ground coffee. After the reservoir has been filled with cold water and the heating element has been switched on the water is pumped from the reservoir to the coffeepot. The cold water cools the flow pipe, as a result of which the thermostat keeps the heating element turned on continuously. As soon as all the water has passed through the temperature of the flow tube rises until the thermostat cuts off the power supply to the heating element. The flow phase or pumping phase is now terminated and the keep-warm phase now begins, in which the thermostat alternately turns the heating element on and off to keep the coffee brew warm. As the scale deposit in the flow pipe increases the heat transfer to the water in the pumping phase decreases. As a result of this, the temperature of the flow pipe rises above the response temperature of the thermostat. The thermostat turns off the heating element until the flow pipe has again cooled down sufficiently by the cold water. The result is that in the case of scale deposit in the flow pipe the thermostat already exhibits a switching behavior in the pumping phase. To some extent this is not a drawback. It merely takes some more time until all the cold water has been pumped off. However, it is known to use the fact that the heating element is turned off for the first time as an indication that the pumping phase has finished. The user is then given a visual or acoustic signal that the coffee is ready. This system will fail in the case of scale deposit in a flow pipe. This is because the thermostat already begins to switch when all the cold water has not yet been pumped off. In the coffee maker known from the afore-mentioned European Patent Application this problem is tackled by comparing the off time of the heating element with an empirically established fixed time period which is characteristic of the coffee maker. On the basis of the comparison it is decided whether the coffee maker is in the pumping phase or in the keep-warm phase and action is taken in accordance with the detected phase.
A drawback of this known coffee maker is that the off time differs from apparatus to apparatus, which renders the decision about the pumping phase or keep-warm phase rather unreliable and which causes inconvenience to the user.